This project provides information that complements the data obtained in Projects 1 and 2 in humans and Projects 2, 3 and 4 in mice. This proposal hypothesize that the effects of aging on the processing of the central auditory nervous system will be manifested as changes in the temporal processing of stimuli and in the ability to process signals in noise. We propose that degradations in temporal processing with age reflect changes in the time course of synaptic events. Specifically, the slowing of synaptic events leads to the increases in the latencies and perhaps decreases in the amplitudes of the auditory evoked potentials due to a greater dispersion of post-synaptic potentials. From our auditory brainstem responses (ABR) data, it appears that temporal processing and simultaneous masking share a mechanism that appears to be neural (specifically, synaptic) in nature, and hence temporal dependencies of the ABR, such as rate and forward masking paradigms, may stress the same underlying processes that are stressed by an ipsilateral-direct masking paradigm. We will investigate the effects of aging in normal hearing young and old human adults using the ABR as the response measure. We will similarly test young-adult and middle-aged C57 mice, and young-adult, middle-aged and old CBA mice. The response measures will include both the ABR and near-field potentials recorded from the inferior colliculus. Using stimulus paradigms that stress synaptic mechanisms (e.g., rate, forward- masking and simultaneous-masking paradigms) and using dependent variables that are insensitive to peripheral hearing loss but sensitive to central auditory nervous system changes (e.g., the I-V interval in the human ABR), then it should be possible to isolate these measures that are sensitive to aging of the CANS.